In subsolvus granites
, which crystallisation occurs at great water pressures, two types of feldspar are present as opposed to hypersolvus
granites in which crystallization at relatively low water pressures results in the formation of a single feldspar
The two types of feldspar are usually near from, respectively:
(1) plagioclase i.e. member of the Anorthite-Albite series (=CaAl2Si2O8-NaAlSi3O8)
(2) alkaline feldspar i.e. member of the Orthoclase-Albite series (=KAlSi3O8-NaAlSi3O8).